1. Field of the Invention
This invention relates to monitoring systems for monitoring the operation of a device, and more particularly to self-diagnosing sensors used in such systems.
2. Related Art
In the ever-increasing competition in the industrial field, industrial equipment, such as rotating machinery, must operate at or near full capacity and sustain such operation for long periods of time. With this type of demand placed on such equipment, periodic maintenance to avoid a catastrophic failure becomes important. Of course, periodic preventative maintenance requires that the equipment be taken off-line for service, thereby potentially resulting in unnecessary down time. Maintenance engineers have been challenged to establish proper time intervals for scheduled preventative maintenance in order to reduce such unnecessary down time.
Alternatively, some maintenance engineers have concluded that the equipment should operate until catastrophic failure. This stems from the fact that, in some instances, it may be better to operate equipment until it fails than to accept the maintenance and the resulting penalty costs of shutting down the equipment prematurely. Also in lieu of scheduled maintenance, some defects may be found by a trained operator. Because such detection is subject to human interpretation, pass/fail criteria may vary between operators and also from day to day with the same operator. Other defects may not be detected at all.
Attempts have been made to automatically monitor such equipment for defects through the use of a sensing element disposed within the equipment itself or through the use of a hand-held device which is periodically attached to one or more discrete locations on the machine being monitored. More sophisticated monitoring systems are permanently installed and carry out essentially continuous monitoring of a machine-mounted transducer along with computer-based analysis of all monitored data.
Most automatic monitoring systems typically sense vibration or temperature. Vibration is produced by the moving parts in the rotating machinery due to causes such as unbalance, misalignment of shafts, worn out bearings, broken gear teeth or foreign particles lodged within the machine. Excessive levels of vibration indicate malfunction of the machine, which can result in machine failure. The temperature of a bearing, for example, can also be monitored to detect the occurrence of over-heating. In some instances, the oil level in the machine may be monitored, automatically through the use of a float system or manually through the use of a dipstick or a sight glass, so that the likelihood of defects or malfunction of the device due to low oil level may be reduced. Other automatic means to detect oil level include beam techniques that measure time of flight or frequency modulation of an ultrasonic, microwave or light/laser beam. Electrical methods have also been employed that detect changes in current, voltage, capacitance or inductance of the liquid to determine the fluid level.
The ability of any conventional monitoring system to detect failures or monitor potential failures is limited by the integrity of the sensors. Conventional monitoring systems generally rely upon an assumption that the sensors are properly functioning when a determination as to the state of the device being monitored may be made. However, this unsupported assumption may lead to false indications that the device being monitored has or is about to malfunction, which has the negative effect of reducing operator reliance on the monitoring system. Malfunctioning sensors may also fail to detect a system malfunction.
One feature of the present invention is a method and apparatus for determining whether the sensors used in a device monitoring system is properly functioning before rendering a determination as to a defect within the device itself. As a result, maintenance costs may be reduced while limiting the number of false indications of failure, thereby increasing the reliability of the monitoring system.
In one particular aspect of the invention, the method includes the steps of receiving a signal from a sensor and a sensor channel representing a noise threshold, receiving a signal from the sensor and the sensor channel when the sensor is energized to obtain a sensed value, and determining whether the sensor is operational based on whether the sensed value exceeds the noise threshold.
In one embodiment, the signal from the sensor and the sensor channel representing the noise threshold may be obtained by receiving a signal from a sensor and a sensor channel when the sensor is de-energized. Alternatively, the noise threshold signal may be stored in memory.
In another embodiment, signals from a plurality of sensors and a sensor channels may be received. When more than one sensor is determined to be malfunctioning, statistical analysis may be performed to determine a malfunctioning sensor. Example of such analyses may be a mean-time-to-failure analysis or statistical trending analysis.
In another embodiment, rather than base a determination on a single sensed value, an average value of a plurality of signals may be generated.
In another aspect of the invention, the method may be performed by a processing unit of a self-diagnostic system.
In another aspect of the invention, the self-diagnostic system may be included in a device monitoring system. The device monitoring system may include a plurality of sensors.
Various embodiments of the present invention provide certain advantages and overcome certain drawbacks of the conventional techniques. Not all embodiments of the invention share the same advantages and those that do may not share them under all circumstances. Further features and advantages of the present invention, as well as the structure and operation of various embodiments of the present invention, are described in detail below with reference to the accompanying drawings.